In the US, I think Denver has a pretty amazing connection to downtown. The airport has a beautiful station and it's nice that a commuter rail system has 15 minute headways and makes few stops to get downtown despite the airport being so far away. I like Minneapolis' connection too. Chicago would be better if there were more room for luggage on the trains, but the ease of getting to the EL isn't bad. Midway is kind of a long walk, but once you do that it's only about a 20-25 minute ride to the loop. O'Hare is easier to walk to the Blue Line, but it's a long ride to downtown with so many stops. I really wish Metra would add some express trains to Union Station once the people mover to complete to the new rental car facility.

I really like airports that have direct access to their intercity/high speed rail network. It's so nice to just board a real train and go directly to a different city without having to cram your luggage onto a cramped subway to get to the city center. Boarding an ICE train right at Frankfurt Airport can't be more convenient.